In known types of nuclear power reactors, for example in a boiling water reactor (BWR) as used in the Dresden Nuclear Power Station near Chicago, Illinois, the reactor core comprises a Plurality of spaced fuel assemblies arranged in an array capable of self-sustained nuclear fission reaction. The core is contained in a pressure vessel wherein it is submerged in a working fluid, such as light water, which serves both as coolant and as a neutron moderator. Each fuel assembly comprises a removable tubular flow channel, typically of approximately square cross section, surrounding an array of elongated, cladded fuel elements or rods containing suitable fuel material, such as uranium or plutonium oxide, supported between upper and lower tie plates. The fuel assemblies are supported in a spaced array in the pressure vessel between an upper core grid and a lower core support. The lower tie plate of each fuel assembly is formed with a nose piece which fits in a support socket for communication with a pressurized coolant supply chamber. The nose piece is formed with openings through which the pressurized coolant flows upward through the fuel assembly flow channels to remove heat from the fuel elements. A typical fuel assembly of this type is shown, for example, by B. A. Smith et al in U.S. Pat. No. 3,689,358. An example of a fuel element or rod is shown in U.S. Pat. No. 3,378,458.
Additional information on nuclear power reactors may be found, for example, in "Nuclear Power Engineering", M. M. El-Wakil, McGraw-Hill Book Company, Inc., 1962.
A typical fuel assembly is formed, for example, by an array of spaced fuel rods supported between upper and lower tie plates, the rods being several feet in length, on the order of one-half inch in diameter and spaced from one another by a fraction of an inch. To provide proper coolant flow past the fuel rods it is important to maintain the fuel rods in fixed spaced relation and restrain them from bowing and vibrating during reactor operation. A plurality of fuel rod spacers positioned in spaced relation along the length of the fuel assembly are provided for this purpose. Such spacers are shown, for example, by B. Matzner et al in U.S. Pat. No. 4,508,679.
In a typical BWR the fuel assemblies are spaced apart, as shown for example by J. R. Fritz et al in U.S. Pat. No. 3,802,995. This leaves gaps or channels between fuel assemblies which are filled with relatively cool water-moderator. Thus the peripheral fuel rods of the fuel assemblies are exposed to neutrons of relatively low thermal energy which are more likely to cause fission in the fuel whereas the fuel rods in the inner region of the fuel assemblies are exposed to neutrons of higher thermal energy.
Also, boiling in the upper part of the BWR core reduces neutron moderation in the upper regions of the fuel assemblies.
Because they are exposed to a relatively greater amount of low energy thermal neutrons the peripheral fuel rods tend to produce relatively more power and hence have higher heat flux than the fuel rods in the inner region. To aid cooling of the peripheral fuel rods and mixing of the water flow through the fuel assembly, inwardly bent flow deflecting tabs can be added to the peripheral support band of the fuel rod spacers. Such vanes are shown, for example, by R. J. Creagan et al in U.S. Pat. No 4,061,536.
Unequal moderator-to-fuel ratio can be alleviated by use of "part-length" fuel rods which extend only over the lower (unvoided) region of the fuel assembly as discussed, for example, by S. Untermyer in U.S. Pat. No. 2,998,367.
To alleviate the unequal neutron moderation caused by the water channels surrounding each fuel assembly and the boiling in the upper region of the fuel core, one or more central fuel rods can be replaced by water conducting tubes which convey unvoided water to the upper region of the fuel assembly. Such arrangements are shown, for example, in the previously mentioned U.S. Pat. No. 3,802,995 and by T. G. Dunlap et al in U.S. Pat. No. 4,420,458. In these two patents the water tubes are of about the same diameter as that of the fuel rods.
The idea of using a large water tube (e.g. one which replaces four fuel rods) is also known as shown, for example, by J. M. West et al in U.S. Pat. No. 3,132,076 (FIG. 5) and by B. Fredin in U.S. Pat. No. 3,808,098.
However, use of such a large water tube presents problems not addressed or solved by the prior art. Such a large water tube is inherently quite rigid (as compared to the smaller diameter fuel rods). Thus if such a large water tube is secured to the lower tie plate, it constitutes a relatively rigid member traversing the fuel assembly through passages in the fuel rod spacers.
If the fuel assembly is subjected to transverse loads such as during a seismic event it is generally assumed that the lower tie plate and nose piece of the assembly remains seated in its support socket while the upper portion of the assembly may experience lateral displacement. In such a case, the relatively stiff large water tube can subject the fuel rod spacers to lateral loads beyond the capability of the spacers (particularly the lower most spacer).
It is an object of the invention to provide practical arrangements for use of a large water tube in a nuclear fuel assembly.
Another object is an improved nuclear fuel assembly which tends to equalize neutron moderation, improve heat transfer and minimize coolant pressure drop.
Another object is a large water tube mounting arrangement which avoids excessive lateral loads on the fuel rod spacers.